RTD Advantages and Disadvantages

19 Field Instrumentation GEN Rev)
• Within its range it is more accurate and has higher sensitivity than a thermocouple.
• In an installation where long leads are required, the RTD does not require special extension cable.
• Unlike thermocouples, radioactive radiation (beta, gamma and neutrons) has minimal effect on RTDs since the parameter measured is resistance, not voltage.
Disadvantages:

• Because the metal used fora RTD must be in its purest form, they are much more expensive than thermocouples.
• In general, an RTD is not capable of measuring as wide a temperature range as a thermocouple.
• A power supply failure can cause erroneous readings Small changes in resistance are being measured, thus all connections must be tight and free of corrosion, which will create errors.
• Among the many uses in a nuclear station, RTDs can be found in the reactor area temperature measurement and fuel channel coolant temperature.

Failure Modes
• An open circuit in the RTD or in the wiring between the RTD and the bridge will cause a high temperature reading.
• Loss of power or a short within the RTD will cause a low temperature reading.

20 Field Instrumentation GEN Rev)


1.5 THERMISTOR

Its name is derived from thermally sensitive resistor. Resistance of a thermistor varies as a function of temperature. Thermistor is an electrical device made of solid semiconductor with high temperature coefficient of resistivity. It is usually made of complex metal oxides (manganese-nickel, manganese – nickel iron. In negative coefficient thermistor resistance decreases as temperature increases. A positive coefficient thermistor (resistance increases as temp. increases) is connected in series with a control relay to react to temperature limits or overload conditions. As temperature increases the resistance of thermistor increases. This causes current in the relay coil to decrease, and relay trips out. It is used to protect industrial motors.
Thermistor is very sensitive so it often does not require wheat stone bridge. They are used in range of -260 to 315 o
C.

21 Field Instrumentation GEN Rev)

Download 3.37 Mb.

Share with your friends: